The present invention relates to a power supply circuit for an internal combustion engine and more particularly to a circuit for separately supplying power to an electronic system for controlling the engine and to loads such as lamps.
In a small engine for a vehicle such as a snowmobile, a magneto for a capacitor discharge ignition (CDI) is adapated to generate alternating current for the loads such as the lamps and a heater of the vehicle. Furthermore, in a vehicle having a battery, the magneto supplies power to charge the battery.
Usually, the magneto is provided in a two-cycle engine which is equipped with an electronic control system for controlling various components of the engine, such as fuel injectors. Japanese Patent Application Laid-Open 63-255543 discloses a fuel injection control system in which the CDI is operated by the magneto.
There are two methods for supplying the power from the magneto to the loads.
(1) As shown in FIG. 2, alternating current generated by a magneto 50 is regulated by a voltage regulator 51 to a predetermined voltage. The regulated voltage is rectified to direct current by a rectifier 52 which is connected to electric loads 53 such as lamps and a heater, to a battery 54, and to an electronic control unit 55.
(2) As shown in FIG. 3, alternating current generated by the magneto 50 is regulated by the voltage regulator 51 to a predetermined voltage which is fed to the loads 53, while the alternating current is rectified to the direct current voltage by a rectifier 56 (half-rectification) which is connected to the battery 54 and to the electronic control unit 55.
The output voltage of the magneto 50 changes in accordance with engine speed. When the engine speed is low, the output voltage is low. If the power is consumed by the electric loads 53 in a low voltage state, the battery 54 is not charged, but discharged to the contrary. In order to solve the problem, another battery having a large capacity must be provided, which causes increases of weight of the vehicle and space for equipping the battery.
For safety of the snowmobile, a headlight thereof is turned on during an operation even in the daytime because of safety. Furthermore, a heater such as a hand warmer is often used. As a result, the battery 54 is not sufficiently charged, so that the operation of the electronic control unit 55 becomes unstable, causing malfunction of the engine.